Conventional high-speed optical modules use lasers to transmit user data through an optical link. For those modules that control laser power with a local control loop, the lasers are monitored locally at a transmit end by local photodiodes to ensure that the output power of the lasers remain within a target range of operation. The local photodetectors are additional hardware that duplicate detectors at a remote (receive) end of the optical link. For modules not implementing a feedback loop, the lasers are factory preset to a predetermined power level and are at an end-of-life when degraded to a point where error-rate requirements are not being meet.
Implementation of a local photodetector for each laser increases the space claim and physical placement complexity within the optical module, the cost of the optical module, and the power dissipation of the optical module as compared to an implementation without the local photodetectors. In addition, the local photodetectors do not provide feedback as to the power needed to have a reliable link. Therefore, the lasers are operated at a maximum allowed power for the specific environment regardless of the optical power reaching the receive end.
In short optical links, where link related loses are minimal, the receive end of the optical link can feed back the level of received power as status information and allow the source to reduce power while still maintaining a valid link and meeting error rate requirements. However, a mechanism must be provided to return the power level status information from the receive end to the power control function at the transmit end. For optical links that make use of remote power and link status reporting, the status information is commonly transmitted through an alternate communications link or mixed with the normal user data traffic carried through the optical link.
Implementing the alternate communications links involves a separate electrical or optical connector at each end of the optical link and separate media paralleling the optical link to carry the status and other maintenance information. The alternative link is not commonly supported by an existing optical cable infrastructure. Therefore, in a single link system, the cost of the additional link can easily double the cost of the original link.
Mixing the status information with the user data results in some loss of the normally available user bandwidth. Furthermore, the user protocol must be modified to support the transport of the embedded status information (which is commonly not supported by standardized communication protocols). Mixing user and nonuser data is also commonly used for other laser safety protocols that determine only the presence or absence of optical power and do not interpret the content of the modulated bit stream.
Referring to FIG. 1, a block diagram of a conventional four-lane optical system 10 is shown. The conventional system 10 includes an optical transceiver module 12a–b at each end of an optical cable 14 having twelve fibers 16a–l. Transceiver module 12a has four laser diode sources 18a–d and four photodetectors 20a–d. Transceiver module 12b has four laser diodes 18e–h and four photodetectors 20e–h. Fibers 16a–d are used to transmit from the optical module 12b to the optical module 12a. The fibers 16i–l are used to transmit in the other direction. The four fibers 16e–h in the middle of the optical cable 14 are unused.